If you are asking if you can convert a single email into PDF, If you copy and pasted or opened it in a PDF app reader and saved it in that format it would be PDF but I would copy it keeping the original.

I'm personally unaware of a way to convert an email message to .pdf format on the iPhone and I'm unaware of any email to .pdf converting apps, but that's not to say that there aren't any. Until a solution is found, convert the email to .pdf format using a computer.